Well, my dark/dark stalker ist kind of PvEish. He´s got Flight as travel power (and air sup as a prerequisite) and is now at level 35 without any major problems so far.
I was a little bit worried about mayhem missions when I built him, but he´s really good at those. Stackable fear is a very nice thing in PvE and PvP.
Only problem - as for all stalkers - is that damn hide hide-breaking auto-aggro and some enemies like Luminary (who sees you from a distance of 50m and is thus quite hard to solo).
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
well i soloed Recluse on my nin/nin and i hear good things about em/regen for PVE also nin/sr works well.
dark/dark hmm i think you'll struggle in the 40+ game.
Defiant's GoEH/MILITIS METUS
I found my Claws/Regen stalker much more effective PvE and PvP than my Dark/Dark, just from a damage output point of view. I doo miss the fear a little I'll admit that, but then I find focus an awesome power and of course the regen healing means I can scrap for much much longer too, which is more fun IMHO.
However it's all personal choice and I'll be the first to admit my Dark/Dark build may not be the best it can be as it was my first toon and I've never respecced.
To be honest I think most stalker builds stand up PvE wise pretty well from what I've heard. We are quoted along side MMs as the soloing class after all.
True. If you want to scrap a lot dark/dark will have problems (damage- and endu-wise), but if you are aware of your strengths and weaknesses you can build quite a powerful stalk-o-troller who is very hard to kill, especially for those stupid AI controlled enemies.
Up to now anything that was doable with my recently deceased claws/nin stalker was as doable (if not easier) with my dark/dark. So, at some point it all comes down to playstyle.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
Claws/SR FTW - have been playing on Relentless since lvl 35 and only have to change down when I meet a longbow Balista 
/dark has no lvl 38 power like retsu and elude and you will struggle against all the AV/EB's in the 40-50's.
Defiant's GoEH/MILITIS METUS
I was wondering, is there any Stalker builds that function particulary well, well enough to be referred to as a PvE Machine, orare they best just left alone unless I want to PvP? (Which I don't, I wanna revel in the untouched content! In style, preferably
)